U.S. patent number 5,111,071 [Application Number 07/681,439] was granted by the patent office on 1992-05-05 for threshold detection circuit.
This patent grant is currently assigned to Texas Instruments Incorporated. Invention is credited to Steven C. Jones, Stephen C. Kwan.
United States Patent |
5,111,071 |
Kwan , et al. |
May 5, 1992 |
Threshold detection circuit
Abstract
A threshold detection circuit (10) is provided which comprises
an input node (12) and an output node (14). The input node (12) is
coupled to a current mirror (22) through a resistor (32). The
current mirror (22) is further coupled to another current mirror
(16), which is arranged to receive a reference current proportional
to absolute temperature. The reference current is mirrored and,
having been increased by a multiplier is received by a current sink
(28). When the voltage level at the input node (12) exceeds a
predetermined voltage threshold level, the current exceeding the
amount sinkable by the current sink (28) is directed to the base of
a switching transistor (30) coupled to the output node (14), and
produces an output voltage level at the output node (14) indicative
of the threshold voltage level being reached an/or exceeded at the
input node (12). The predetermined threshold voltage may be at a
level exceeding the circuit supply voltage level and is independent
of ambient operating temperature.
Inventors: |
Kwan; Stephen C. (Plano,
TX), Jones; Steven C. (Garland, TX) |
Assignee: |
Texas Instruments Incorporated
(Dallas, TX)
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Family
ID: |
27026154 |
Appl.
No.: |
07/681,439 |
Filed: |
April 3, 1991 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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423824 |
Oct 19, 1989 |
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Current U.S.
Class: |
327/77;
327/83 |
Current CPC
Class: |
G01R
19/16576 (20130101) |
Current International
Class: |
G01R
19/165 (20060101); H03K 005/153 () |
Field of
Search: |
;307/350,362,363 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
D M. Taub, "Current-Threshold Circuit", IBM Technical Disclosure
Bulletin vol. 17, No. 3, Aug. 1974, p. 750..
|
Primary Examiner: Zazworsky; John
Attorney, Agent or Firm: Barndt; B. Peter Donaldson; Richard
Hiller; William E.
Parent Case Text
This application is a continuation of application Ser. No.
07/423,824, filed Oct. 19, 1989, now abandoned.
Claims
What is claimed is:
1. A threshold detection circuit comprising:
an input node for receiving an input voltage;
a first circuit coupled to a reference current;
a second circuit coupled to said first circuit, said second circuit
producing a first current proportional to said reference current,
wherein said first current is larger than said reference
current;
a current sink coupled to said first current for sinking a portion
of said current;
a resistive element coupled between said node and said second
circuit; and
an output circuit coupled to said current sink, said output circuit
providing an output voltage in response to said input voltage
exceeding a predetermined threshold.
2. The threshold detection circuit, as set forth in claim 1,
wherein said switching circuit is coupled to said output mode, said
switching circuit being responsive to said first current having a
magnitude larger than the current sinkable by said current
sink.
3. The threshold detection circuit, as set forth in claim 1,
wherein said switching circuit includes a first transistor having a
base, a collector and an emitter, the base thereof coupled to said
current sink and arranged to receive a portion of said first
current.
4. The threshold detection circuit, as set forth in claim 3,
wherein said switching circuit further comprises a second
transistor having a base, a collector and an emitter, the collector
thereof coupled to said output node, and the base thereof coupled
to the collector of said first transistor, said second transistor
being responsive to said first transistor receiving said portion of
said first current and producing an output voltage at said output
node.
5. The threshold detection circuit, as set forth in claim 1,
further comprising at least one transistor having a base, a
collector and an emitter, said collector and said base coupled to
said second circuit and said emitter coupled to said current
sink.
6. The threshold detection circuit, as set forth in claim 5,
wherein said transistor base is coupled to said collector
thereof.
7. The threshold detection circuit, as set forth in claim 5,
further comprising:
a first resistor coupled between said base and collector of said
transistor; and
a second resistor coupled between said base and emitter of said
transistor.
8. The threshold detection circuit, as set forth in claim 1,
wherein said reference current is proportional to absolute
temperature.
9. The threshold detection circuit, as set forth in claim 1,
wherein said predetermined threshold is proportional to a
resistance of said resistive element.
10. The threshold detection circuit, as set forth in claim 1,
wherein said input node exhibits high input impedance
characteristics in response to the reference current being less
than a predetermined value.
11. A threshold detection circuit comprising:
an input node;
a first current mirror for receiving a reference current and
generating a first current having a magnitude of a first
predetermined multiple of said reference current;
a second current mirror coupled to said first current mirror and
generating a second current having a magnitude of a second
predetermined multiple of said first current;
a current sink coupled to said second current mirror and arranged
to sink a predetermined amount of said second current;
a resistor coupled between said input node and said second current
mirror; and
a switching circuit coupled to said second current mirror and said
current sink, said switching circuit being responsive to said input
node having at least a predetermined threshold voltage level.
12. The threshold detection circuit, as set forth in claim 11,
wherein said first current mirror comprises:
a first transistor having a base, an emitter and a collector, said
base and collector coupled together;
a second transistor having a base, an emitter and a collector, said
base coupled with said base of said first transistor; and
said second transistor emitter area being a first predetermined
multiple of the area of said first transistor emitter.
13. The threshold detection circuit, as set forth in claim 12,
wherein said second current mirror comprises:
a third transistor having a base, an emitter and a collector, said
base and collector coupled together, and said collector coupled to
said second transistor collector;
a fourth transistor having a base, an emitter and a collector, said
base coupled with said third transistor base; and
said fourth transistor emitter area being a second predetermined
multiple of the area of said third transistor emitter.
14. The threshold detection circuit, as set forth in claim 12,
wherein said current sink comprises a third transistor having a
base, an emitter and a collector, said third transistor base
coupled with said first and second transistor bases.
15. The threshold detection circuit, as set forth in claim 11,
further comprising at least one transistor having a base, a
collector and an emitter, said transistor coupled between said
second current mirror and said current sink.
16. The threshold detection circuit, as set forth in claim 15,
wherein said transistor base is coupled to the collector
thereof.
17. The threshold detection circuit, as set forth in claim 15,
further comprising:
a first resistor coupled between said base and collector of said
transistor; and
a second resistor coupled between said base and emitter of said
transistor.
18. The threshold detection circuit, as set forth in claim 11,
wherein said switching circuit includes a first transistor having a
base, a collector and an emitter, the base thereof coupled to said
current sink and arranged to receive a portion of said second
current.
19. The threshold detection circuit, as set forth in claim 18,
further comprising a second transistor having a base, a collector
and an emitter, the collector thereof coupled to the collector of
said first transistor, and the emitter thereof coupled to a supply
voltage.
20. The threshold detection circuit, as set forth in claim 19,
further comprising a third transistor having a base, a collector
and an emitter, the base thereof coupled to the collector of said
first transistor, and said output node coupled to the collector of
said third transistor.
21. The threshold detection circuit, as set forth in claim 11,
wherein said reference current is proportional to absolute
temperature.
22. The threshold detection circuit, as set forth in claim 11,
wherein said threshold voltage level is adjustable by varying the
value of said resistor.
23. The threshold detection circuit, as set forth in claim 11,
being operable to detect a threshold voltage level exceeding a
circuit supply voltage level.
24. The threshold detection circuit, as set forth in claim 11,
wherein said input node exhibits high input impedance
characteristics in response to said reference current being less
than a predetermined value.
25. A threshold detection circuit comprising:
an input node for receiving an input voltage;
a first resistor coupled in series with said input node;
a first current mirror arranged to receive a reference current and
to reproduce and sink a first current having a magnitude of a
multiple of said reference current;
a second current mirror coupled to said first current mirror and
adapted to reproduce and source a second current having a magnitude
of a multiple of said first current magnitude, said second current
mirror coupled with said resistor;
a first transistor having a base, a collector and an emitter, said
collector coupled with said second current mirror, said first
transistor adapted to sink a predetermined amount of said second
current;
a second transistor having a base, a collector and an emitter, said
base coupled to said first transistor collector and adapted to
receive any of said second current in excess of current sinkable by
said first transistor;
a third transistor having a base, a collector and an emitter, said
base coupled with said second transistor collector; and
an output node coupled to said third transistor collector, said
output node providing an output voltage responsive to said input
voltage exceeding a predetermined threshold.
26. A method for detecting a threshold, said method comprising the
steps of:
supplying an input voltage to an input node;
providing a resistive element coupled to said input node;
receiving a reference current;
producing a first current which is a predetermined multiple of said
reference current;
drawing a second current through said resistive element which is a
predetermined multiple of said first current;
sinking a predetermined portion of said second current;
driving a switching circuit with a portion of said first current in
excess of said sink portion; and
producing an output voltage at an output node coupled to said
switching circuit, said output voltage being responsive to said
input voltage exceeding said threshold.
27. The method, as set forth in claim 26, wherein said step of
providing a resistive element comprises selecting a resistance
value whereby said threshold is proportional to said resistance
value.
28. A method for constructing a threshold detecting circuit, said
method comprising the steps of:
providing an input node having an input voltage;
providing a resistive element coupled to said input node;
forming a first circuit for receiving a reference current and
producing a first current proportional to said reference current in
magnitude;
forming a second circuit coupled to said resistive element for
receiving said first current and drawing a second current through
said resistive element proportional to said first current in
magnitude;
forming a current sink arranged to sink a portion of said second
current; and
providing an output node coupled to said current sink, said output
node providing an output voltage responsive to said input voltage
exceeding a predetermined threshold.
Description
TECHNICAL FIELD OF THE INVENTION
This invention relates in general to the field of integrated
electronic devices. More particularly, the present invention
relates to an apparatus and a method for detecting a threshold
voltage.
BACKGROUND OF THE INVENTION
Many threshold detecting devices are available today. Typically,
threshold detection functions have been accomplished through the
use of a differential comparator circuit. Such threshold detecting
circuits are limited in their capabilities which correspondingly
restricts their usage.
One desirable feature of a threshold detection circuit is the
independence of the voltage threshold level with respect to ambient
temperature. Most integrated electronic devices are designed to
operate within a well defined operational temperature range.
To comply with the temperature independence requirement, more
complicated circuits may be included in addition to the
differential comparator circuit. With the added circuits to satisfy
the above requirements, the threshold detection circuit becomes
needlessly complex and cumbersome.
Another desirable feature of a threshold detection circuit is the
ability to detect a threshold voltage substantially above the
operating supply voltage.
In applications where the desired threshold voltage is
substantially larger than the supply voltage, an auxiliary circuit,
such as a resistor network, is typically required to step down the
input voltage to a level below the supply voltage. This additional
circuitry increases the size of the threshold detection
circuit.
Accordingly, a need has arisen for a threshold detection circuit
which has relatively few components, can maintain a substantially
constant voltage threshold over a wide temperature range, and can
detect a threshold voltage in excess of the supply voltage.
SUMMARY OF THE INVENTION
In accordance with the present invention, a threshold detection
circuit is provided which substantially eliminates or reduces
disadvantages and problems associated with prior threshold
detection circuits.
In one aspect of the present invention, a threshold detection
circuit is provided. The circuit includes an input node for
receiving an input voltage and a current sink coupled to the input
node. A resistive element, which determines the threshold to be
detected, is further coupled to the input node. An output voltage
will be provided at an output node which is responsive to the input
voltage exceeding a predetermined threshold.
In another aspect of the present invention, a threshold detection
circuit is provided, comprising an input node for receiving an
input voltage, a first circuit coupled to a reference current, a
second circuit coupled to the first circuit, the second circuit
producing a first current proportional to the reference current, a
current sink coupled to the first current for sinking a portion of
the first current, a resistive element coupled between the input
node and the second circuit and an output circuit coupled to the
current sink, the output circuit providing an output voltage in
response to the input voltage exceeding a predetermined
threshold.
In another aspect of the present invention, the circuit includes a
first current mirror to receive a reference current and to
reproduce and sink a first current having a magnitude of a multiple
of the reference current magnitude. A second current mirror is
arranged to reproduce and source a second current having a
magnitude of a multiple of the first current magnitude. Coupled
with the second current mirror is a first transistor having a base,
a collector and an emitter. A second transistor having a base, a
collector and an emitter is coupled to the first transistor and
adapted to receive any of the second current in excess of current
sinkable by the first transistor.
In yet another aspect of the present invention there is provided a
method for detecting a voltage threshold. The method comprises the
steps of supplying an input voltage to an input node, providing a
resistance at the input node, receiving a reference current, and
producing a first current exceeding the reference current by a
first predetermined amount. The method further includes the steps
of sinking a predetermined portion of the first current. A
switching element is driven with the first current in excess of the
sinkable portion, and an output voltage is produced at an output
node responsive to the input voltage exceeding a threshold.
In another aspect of the present invention there is provided a
method for constructing a threshold detecting circuit. The method
comprises the steps of forming an input node, forming a current
sink coupled to the input node, forming an output node being
responsive to an input voltage exceeding a predetermined threshold,
and forming a resistive element coupled to the input node.
An important technical advantage of the present invention is that
it provides for a circuit and method for detecting a threshold
voltage at a level exceeding the supply voltage level. Another
technical advantage of the present invention is the insensitivity
of the threshold voltage level to ambient temperature
variations.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the present invention, reference may
be made to the accompanying drawings, in which:
FIG. 1 is a schematic diagram of a threshold detection circuit
constructed in accordance with the present invention;
FIG. 2 is a schematic diagram of one embodiment of the present
invention;
FIG. 3 is a schematic diagram of a portion of an alternative
embodiment of the present invention; and
FIG. 4 is a schematic diagram of a reference current supplying
circuit.
DETAILED DESCRIPTION OF THE INVENTION
With reference to the drawings, FIG. 1 illustrates a schematic
diagram of a threshold detection circuit, indicated generally at 10
and constructed according to the teaching of the present invention.
The circuit 10 comprises an input node 12 and an output node 14.
The input node 12 is arranged to receive an input voltage, the
voltage level of which the circuit 10 is monitoring.
The threshold detecting circuit 10 further comprises a first
current mirror 16 including a first transistor 18 and a second
transistor 20. The first transistor 18 collector is coupled to the
base thereof, and the base is further coupled to the base of the
second transistor 20. The emitters of the first and second
transistors 16, 18 are coupled to one another. The first and second
transistors 18, 20 are constructed such that they exhibit
substantially identical transfer characteristics, with the
exception that the second transistor 20 is sized to be N times
larger than the first transistor 18, where N is a predetermined
factor. The sizing is accomplished, for example, by forming the
second transistor 20 to have an emitter N times larger in area than
the emitter of the first transistor 18. The first and second
transistors 18, 20 are shown to be NPN-type bipolar junction
transistors, but the present invention is also operable with
PNP-type transistors as known in the art.
The first current mirror 16 is arranged to receive a reference
current I.sub.PTAT, and to mirror and sink a current of the
magnitude NI.sub.PTAT. I.sub.PTAT flows into the collector of the
first transistor 18 and NI.sub.PTAT flows into the collector of the
second transistor 20. The N factor of NI.sub.PTAT is due to the
sizing of the first transistor 18 and the second transistor 20.
A second current mirror 22 is coupled to the first current mirror
16 so that the current NI.sub.PTAT is mirrored again. The second
current mirror 22 includes a third transistor 24 and a fourth
transistor 26 with their bases coupled together. The collector of
the third transistor 24 is further coupled to the bases thereof.
The emitters of the third transistor 24 and the fourth transistor
26 are coupled to one another. The third and fourth transistors 24,
26 also have substantially identical transfer characteristics, with
the exception that the fourth transistor 26 has an emitter M times
the size of the third transistor 24. Since the current flowing out
of the collector of the third transistor 24 and into the collector
of the second transistor 20 is NI.sub.PTAT, the current flowing out
of the collector of the fourth transistor 26 is
(M.times.N)I.sub.PTAT, the fourth transistor 26 being M times
larger than the third transistor 24.
A current sink 28 is arranged to receive a portion of the current
(M.times.N)I.sub.PTAT flowing out of the collector of the fourth
transistor 26. The current sink 28 includes a transistor 29 having
a collector connected to the collector of the fourth transistor 26.
The base of the transistor 29 is connected to the base of the first
and second transistors 18, 20. The maximum amount of current
sinkable by the current sink 28 is configured to be I.sub.PTAT. Any
excess current of the magnitude (M.times.N)I.sub.PTAT -I.sub.PTAT
provides a base drive current to a fifth transistor 30. The fifth
transistor 30, when driven by the base current, provides an output
voltage level at the output node 14.
In order for the fifth transistor 30 to switch states, the base
current of the fifth transistor 30 must be large enough to drive
the fifth transistor 30 into the active region. It follows that the
collector current flowing out of the fourth transistor 26 in excess
of the current sinkable by the current sink 28 must provide this
base drive current. For the base of the fifth transistor 30 to
begin to receive current, I.sub.IN coming into the input node 12
must satisfy the sum of the current I.sub.PTAT flowing into the
current sink 28, and the current NI.sub.PTAT flowing into the first
current mirror 16. The threshold voltage level at which the circuit
10 changes state is equal to the sum of the base to emitter voltage
V.sub.BE of the fifth transistor 30, the saturation voltage
V.sub.SAT of the fourth transistor 26, and the voltage drop across
a resistor 32 having the value R.sub.1, as expressed below:
where I.sub.TH is the input current at threshold, and R.sub.1 is
the resistance of a resistor 32 coupled between the input node 12
and the second current mirror 22. As is apparent in Equation (1)
above, the threshold voltage V.sub.TH can be adjusted by changing
the value R.sub.1 of the resistor 32 without changing the threshold
current I.sub.TH.
The threshold current I.sub.TH is defined as the input current when
the base of the fifth transistor 30 begins to receive current.
Therefore, this threshold current is the one branch of the first
current mirror 16:
The current I.sub.PTAT is proportional to absolute temperature in
degrees Kelvin. This temperature dependent current I.sub.PTAT is
created using circuitry known in the art such that its magnitude is
proportional to the junction temperature of the device. As
discussed below, the I.sub.PTAT temperature characteristics
contribute to the invariance of the voltage threshold of the
threshold detection circuit 10.
FIG. 2 illustrates an embodiment of the present invention indicated
generally at 33. The circuit 33 shown may be divided into six
functional circuit blocks for the purpose of discussion. First and
second current mirrors 34, 36 and a current sink 38 are
substantially the same structure as the first and second current
mirrors 16, 22 and the current sink 28 shown in FIG. 1 and
discussed above.
A reference current generator circuit 40 used to generate the
current I.sub.PTAT proportional to absolute temperature. A current
mirror 42 is used to supply the current I.sub.PTAT to the first
current mirror 34. I.sub.PTAT can be expressed in terms of the
absolute junction temperature: ##EQU1## where k is the Boltzman
constant, T is the absolute junction temperature in degrees Kelvin,
q is the unit charge of an electron, N is the ratio of the sizes of
the transistor emitters, and R.sub.2 is the resistance value of a
resistor 44 used in the reference current generator circuit 40.
As shown previously with reference to FIG. 1, the current
I.sub.PTAT generated by the circuit 40 and current mirror 42 is
received by the first current mirror 34. In this embodiment, the
emitter area ratio of the first current mirror transistors 68, 70
is two to one, which induces a current of the magnitude 2I.sub.PTAT
to flow from the second current mirror 36 into the first current
mirror 34. Similarly, the transistor emitter area ratio in the
second current mirror 36 is 0.67 to 1. The current sourced by the
second current mirror 36 is then 2I.sub.PTAT times 0.67, which
equals 1.33I.sub.PTAT. A diode-connected transistor 46 has its base
connected to its collector and is arranged to receive the
1.33I.sub.PTAT from the second current mirror 36. A current sink
38, capable of sinking a current of the magnitude I.sub.PTAT, is
coupled to the emitter of the diode-connected transistor 46. An
output stage 48 is arranged to receive the excess current from the
second current mirror 36 and to provide an output voltage
indicative of threshold voltage detection. The detailed structure
and function of the present embodiment of the invention is
described below.
The reference current generator circuit 40 is connected to a supply
voltage through a first resistor 50. The first resistor 50 is
connected to the collector of a first transistor 52. The collector
and the base of the first transistor 52 are coupled together and to
the base of a second transistor 54. The emitter of the first
transistor 52 is also coupled to the collector of a third
transistor 56 and to the base of a fourth transistor 58. The base
of the third transistor 56 is coupled to the collector of the
fourth transistor 58, which is also coupled to the emitter of the
second transistor 54. The emitter of the fourth transistor 58 is
coupled to a resistor 44 having a resistance value R.sub.2. The
fourth transistor 58 is sized to be eight times the size of each of
the first, second and third transistors 52, 54, 56. The resistor 44
value R.sub.2 plays an important role in determining the threshold
voltage level detected by circuit 33.
A current having a magnitude I.sub.PTAT is generated by the
reference current generator circuit 40 and mirrored by a current
mirror 42. The current mirror 42 includes a fifth transistor 60
with its emitter coupled to a supply voltage. The collector of the
fifth transistor 60 is coupled to the collector of the second
transistor 54 of the reference current generator circuit 40. The
base of a sixth transistor 62 is coupled to the base of the fifth
transistor 60, and is further coupled to a supply voltage through a
resistor 64. The resistor 64 is further coupled to the emitter of a
seventh transistor 66, which has a base connected to the collector
of the fifth transistor 60 and a collector connected to ground.
Therefore, the current generated by the reference current generator
circuit 40 is sourced from the collector of the fifth transistor 60
of the current mirror 42. The collector of the sixth transistor 62
is coupled to the first current mirror 34 of the threshold
detection circuit 33 to provide the reference current
I.sub.PTAT.
The threshold detection circuit 33 as illustrated in FIG. 2 is
comparable to the circuit 10 shown in FIG. 1. The first current
mirror 34 comprises a first transistor 68 and a second transistor
70. The collector and base of the first transistor 68 are coupled
together along with the base of the second transistor 70. The
emitters of the first and second transistors 68, 70 are coupled to
ground. The second transistor 70 is sized to be two times the size
of the first transistor 68 and therefore the current induced in the
collector of the second transistor 70 is 2I.sub.PTAT. Thus, the
second current mirror 36, being coupled to the first current mirror
34, is induced to source a current of the magnitude 2I.sub.PTAT to
the first current mirror 34.
The second current mirror 36 has a first transistor 72 and a second
transistor 74 with their bases coupled together. The base of the
first transistor 72 is also coupled to the collector thereof. The
second transistor 74 is sized to be 0.67 times the size of the
first transistor 72. As previously disclosed, the sizing is by
means of the difference in emitter area sizes. The amount of
current sourced by the second transistor 74 is 0.67 times
2I.sub.PTAT which is equal to 1.33I.sub.PTAT The emitters or both
the first and second transistors 72, 74 are coupled to the input
node 12 through a resistor 75 having a resistance value
R.sub.1.
A transistor 46 is coupled in series to the collector of the second
transistor 74 of the second current mirror 36. The base of the
transistor 46 is connected to the collector thereof, arranged like
a diode. The emitter of the transistor 46 is coupled to a current
sink 38 comprising a transistor 76. The transistor 76 is configured
to receive and sink a current, having a magnitude of I.sub.PTAT,
from the second current mirror 36 and the transistor 46.
The output stage 48 is coupled to the current sink 38 and includes
first, second and third transistors 78, 79, 80. The collector of
the first transistor 78 of the output stage 48 is coupled to the
collector of the second transistor 79. The emitter of the second
transistor 79 is coupled to a supply voltage while the emitter of
the first transistor 78 is connected to ground. The collector of
the first transistor 78 is further coupled to the base of the third
transistor 80. The second transistor 79 base is coupled to the
bases of matched transistors 60, 62 of the current mirror 42. The
collector of the third transistor 80 is coupled to the supply
voltage through a resistor 81 while the emitter thereof is
connected to ground. The base of the first transistor 78 is coupled
to the collector input of the current sink 38 and configured to
receive any excess current from the second current mirror 36 that
the current sink 38 cannot sink.
As is apparent from FIG. 2 and Equation (2), the input current
I.sub.IN reaches threshold value when it is equal to the collector
current of the current sink 38 added to the collector current of
the second transistor 70 of the first current mirror 34. Therefore,
I.sub.TH is the sum of I.sub.PTAT plus 2I.sub.PTAT, which is
3I.sub.PTAT. Any input current exceeding 3I.sub.PTAT is operable to
drive the base of the first transistor 78 of the output stage 48,
which in turn causes the transistor 80 to go into the cutoff region
and thereby giving rise to a change of state of the output node
14.
In the present embodiment of the invention shown in FIG. 2, the
threshold voltage level can be described as the sum of the base to
emitter voltages of the first transistor 78 and the diode-connected
transistor 46, in addition to the saturation voltage of the second
transistor 74 and the voltage drop across the resistor 75:
The saturation voltage of the second transistor 74 of the second
current mirror 36 may be ignored because it contributes negligibly
to the sum. Substituting for I.sub.TH in Equation (4) and assuming
that V.sub.BE1 =V.sub.BE2, the threshold voltage V.sub.TH can be
expressed as: ##EQU2## In this case N=8, which is established by
fourth transistor 58 emitter sizing of the current generator
circuit 40.
It may be appreciated that the temperature characteristics of the
threshold voltage is dependent on the temperature characteristics
of each term to the right of the equal sign in Equation (5). A
transistor's base to emitter voltage temperature characteristic is
fairly predictable, and is typically -2 mV/.degree.C.
Differentiating the expression in Equation (5) with respect to
temperature and setting it to zero yields an equation which may be
solved to indicate where the change in threshold voltage with
respect to temperature is a minimum. The derivative is shown in
equation (6): ##EQU3## Rearranging the above equation, it becomes
apparent that the resistance values R.sub.1 and R.sub.2 of the
resistors 75, 44 in the circuit 33 may be adjusted to compensate
for the temperature variance in the base to emitter voltage of the
transistors 46, 78: ##EQU4##
Given a known resistance value R.sub.2 for resistor 44 and N, an
optimum resistance value R.sub.1 for resistor 75 may be found from
manipulating the expression in Equation (7) that minimizes or
eliminates the temperature dependence of the threshold detection
circuit 33.
As indicated in Equation (4), the threshold voltage level is
determined by the sum of the base to emitter voltage V.sub.BE of
multiple transistors 46, 78, a negligible saturation voltage, and
the voltage drop across resistor 75. Therefore, the threshold
voltage level can be modified by varying the number of base to
emitter voltages V.sub.BE between the second transistor 74 of the
second current mirror 36 and the first transistor 78 of the output
stage 48. In the embodiment illustrated in FIG. 2, only one
V.sub.BE is between those transistors 74, 78. Moreover, an integer
multiple of diode-connected transistors, like transistor voltage
level V.sub.TH. In applications where non-integer multiples of the
V.sub.BE is desired, a circuit, indicated generally at 82 and shown
in FIG. 3, may be used.
Referring to FIG. 3, circuit 82 includes a transistor 84 having a
first resistor 86 connected between its collector and base, and a
second resistor 87 connected between its base and emitter. A node
88 connected to the collector of the transistor 84 may be coupled
to the second current mirror 36, and a node 89 connected to the
emitter may be coupled to the current sink 38. The first resistor
86 has a resistance value R.sub.A and the second resistor 87 has a
resistance value R.sub.B. The multiplier for the V.sub.BE, f, is
equal to the expression in Equation (8): ##EQU5## Thus, both the
first and second resistors 86, 87 may be chosen at desired
resistance values to effect a non-integer effective V.sub.BE.
Referring now to FIG. 4, an alternate embodiment of the current
generator circuit 40 of FIG. 2 is shown. The circuit, indicated
generally at 90, employs PNP-type transistors and may be used to
substitute for both the current generator circuit 40 and the
current mirror 42 shown in FIG. 2. The circuit 90 includes a first
transistor 92 with its base coupled to the collector of a second
transistor 94 and its collector coupled to the emitter of a third
transistor 96. The collector of the second transistor 94 is further
coupled to the emitter of a fourth transistor 98, and the base of
the second transistor 94 is connected to the collector of the first
transistor 92. The collector of the third transistor 96 is coupled
to a resistor 100. The emitter of the second transistor 94 is
coupled to a supply voltage through a resistor 102 of a resistance
value R.sub.2. The second transistor 94 emitter is sized to be
eight times larger than the emitter of the fourth transistor 98. A
current I.sub.PTAT, which is proportional to temperature variances,
is generated by the circuit 90 and flows from the collector of
transistor 98. The circuit 90 generally functions like that of the
current generator circuit 40. Due to the nature of the PNP
transistors, the current mirror circuit 42 may be eliminated, and
the current generator circuit 90 may be coupled directly to the
first current mirror 16 of the threshold detection circuit 10 of
FIG. 1 or the first current mirror 34 of the threshold detection
circuit 33 of FIG. 2.
In summary, it is apparent from examining Equation (4) that the
threshold voltage level is independent of the supply voltage value.
Therefore, the threshold detection circuits constructed in
accordance with the present invention are capable of detecting
voltages far exceeding the supply voltage level. It is also
apparent that by varying the number of diode-connected transistors
connected between the second current mirror 36 and the switching
transistor 78, and further by adjusting the resistance value
R.sub.1 of the resistor 75, the threshold voltage level of the
circuit may be established at a desired level. Thus, the circuit
threshold characteristics may be changed by modifying a relatively
small number of circuit components.
An additional feature of the a circuit under the present invention
is the availability of a high impedance state at its input node 12.
Note that no input current I.sub.IN can flow into the circuit 10
unless the transistors of the second current mirror 22 or 36 are
provided with base current. Therefore, a high impedance state can
be achieved by disabling the NI.sub.PTAT current to second current
mirror 22 or 36. For example, I.sub.PTAT may be disabled or
otherwise redirected to ground in order to disable NI.sub.PTAT
thereby creating a high impedance state.
Other modifications to the circuit provided herein are possible
without departing from the scope of the present invention. For
example, the construction of current mirrors, current sources and
current sinks may be varied according to circuit applications. In
addition, there are a variety of methods of creating the current
proportional to temperature, I.sub.PTAT. The presentation of the
specific embodiment as illustrated in FIG. 2 is solely for the
purpose of teaching important technical advantages of the present
invention and should not be construed to limit the scope of the
present invention.
Although the present invention has been described in detail, it
should be understood that various changes, substitutions and
alterations can be made hereto without departing from the spirit
and scope of the invention as defined by the appended claims.
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